Commercial production of Low Temperature Superconductor composites (LTS) has hitherto enabled the fabrication of magnetic resonance imaging magnets, which provide both for high magnetic field intensity by normal engineering standards, so maximising NMR signal to noise ratio, and for significant field uniformity in a target volume suitable for MRI. The LTS conductors are available in long lengths of wire or cable. These are mechanically self supporting and ductile, such that conventional coil winding techniques are enabled for the fabrication of solenoid coils. In addition, large and complex stresses due to electro magnetic forces, as well as thermal strain are accommodated by LTS conductors. However, the need to cool such magnet windings in operation to the temperature at which Helium is a liquid at atmospheric pressure, thereby providing a refrigeration temperature of approximately 4.2° K, such that LTS materials carry substantial super currents for field creation, is a major economic and deployment constraint of MRI applications.
High temperature superconducting materials (HTS) are desirable as a replacement for LTS materials as they carry electric currents in a superconducting state at temperatures well above liquid Helium refrigeration, in some cases as high as 120° K, which brings more adaptable and rugged magnet systems. Some HTS materials may be conveniently used in the temperature range achieved by deploying nitrogen in gas or liquid form as a refrigerant or heat transfer medium, typically from 66° K upwards. However, HTS materials are constrained in use for MRI magnets by other factors relevant to engineering requirements.
The best materials in terms of carrying super currents at elevated temperatures, and in the presence of magnetic fields, with high current densities are HTS thin films. Unfortunately, complex film deposition processes, such as PVD, MOCVD or sol-gel routes, are used to make laboratory scale examples of these thin film materials, and it has proved problematic to reliably scale up thin film deposition plant for production of continuous lengths of conductor on the scale of LTS conductor without defects causing locally poor superconductivity which then limits the end-to-end current available. Defects can be fundamental in nature, e.g. crystallographic defects, or they can be introduced during the many handling operations necessary for the long lengths. Further, even if long continuous lengths of conductor could be reliably and successfully produced by scaled up film deposition, the winding geometries and assembly procedures desirable for MRI magnets, as used with LTS practice, and the stresses and strains arising in use of a large cold electro-magnets, may cause mechanical failure of ceramic HTS thin films or, at the very least, scratches.
In earlier work, for example US 2007298971 and UK Patent Application No. 0903942.1 (as set out in Appendix 1) it has been shown that HTS thin films deposited on supporting surfaces, preferably closed cylindrical or conical surface geometries, which have defined current paths lithographed on the HTS material, can be used as modular sources of magnetic field, which can then be arranged in plurality in a range of settings to combine their fields in a manner suitable for MRI. This concept provides for modular electro magnets which overcome the difficulties referred to in deploying HTS thin film conductor—known as “coated conductor” for MRI magnets. In particular, the use of supported HTS surfaces with defined continuous conducting paths employing a limited volume of thin film achieves an integrated structure well able to accommodate stresses arising in use. The conducting path stress levels are also reduced for a given MRI field in many cases as the conducting paths are relatively confined, being modular, and the total radius of the MRI magnet is not “brought to bear” on the stress raising associated with electromagnetic forces, assembly and thermal strain. The modules of limited HTS material volume lend themselves to batch production and testing prior to magnet construction, using the most satisfactory deposition methods. Further, electromagnets constructed using arrays of modular field sources, in which module fields are combined in a target region to produce a suitable field profile for MRI, provide the magnet with component redundancy, wherein failed modules may be exchanged without complete de-construction of the magnet.
In UK Patent Application No. 0903942.1 (as set out in Appendix 1) it was shown that the ability to define current paths on a support surface allowed for the creation of a magnetic field within the module, the principal direction of which can be chosen relative to the main geometric axes of the module. The principal direction of magnetic field relative to the geometric axes of the module can be assigned a dedicated relationship, fixed in operation of the module by design of the defined current paths. Modules are arrayed with their magnetic origins located on a curvilinear contour joining said origins by the shortest route, and wholly or partly enclosing the target MRI volume. A MRI magnet is achieved by, first combining the projected fields of modules arrayed along a given array contour, then, secondly, associating a number of arrays such that each array shares an axis of symmetry with neighbouring arrays, and the combined projected fields of associated arrays provide a field in the MRI target region that is more uniform than the projected field of one array alone.
Importantly in attaining uniform MRI target fields, the properties of the magnetic field profile produced by magnet modules can differ between modules along an array contour. Similarly, different types of module are employed in associated arrays. Thus it is desirable to have the freedom to construct and shape the HTS coated surface and the continuous current paths lithographed on the surface with the degree of freedom inherent in the use of modular HTS batch produced sources of magnetic field. Examples are contained in UK Patent Application No. 0903942.1 (as set out in Appendix 1) of preferable relationships between defined continuous current paths on the support surfaces of modules, and assemblies of associated arrays suitable for producing a target MRI field, acting together as a MRI magnet.
Individual magnetic modules may be supplied with electric current individually or in groups using one or more associated power supplies which provide power either by conductive or inductive coupling to the defined continuous current paths. Modules may be connected in series to a power supply, or more than one power supply may be provided to generate a predetermined magnetic field in each module, or groups of said modules within an array, so as to control the size of the target MRI field volume.
The concept described in UK Patent Application No. 0903942.1 (as set out in Appendix 1) for the achieving of MRI quality field using discrete volumes of HTS material, arranged as modules comprising coated support surfaces that provide for defined current paths, overcomes the problem of enabling a precision field magnet suitable for MRI using available HTS thin film production methods. The use of coated support surfaces, generally of a curvilinear form, but typically represented by cylindrical or conical support surface geometries, suffers from the difficulty of providing a large number of field creating Ampere-turns in the limited thickness of one coated layer. In UK Patent Application No. 0903942.1 the use of ferromagnetic material in part of the volume of a module to augment the field of the defined current path was proposed. This method makes efficient use of HTS thin film material, but suffers from the disadvantage that the magnetic moment of the module becomes a non-linear function of current in the defined current path, complicating the control required to project a uniform field. Typically, a sufficient number of Ampere-turns are used to saturate the ferromagnetic material, with an excess of Ampere-turns to provide field control over and above the field created by the “constant” saturation magnetisation of the ferromagnetic material.
UK Patent Application No. 0903942.1 (as set out in Appendix 1) also describes the concept of “a plurality of magnet modules generating a magnetic field, wherein the associated magnetic fields of the modules combine at a target volume to create a field of uniformity sufficient for MRI, relying on each module comprising a support surface for a layer of superconductor through which an electric current is caused to flow in defined current paths”.
It has also been proposed that multiple layers of HTS thin film material may be deposited, interleaved with electrical insulating oxide materials, such that multiple defined current paths could be provided in one field creating module, which would overcome this problem, as disclosed in, for example, US Patent Publication No. 2007298971. However, this approach requires the addition to the deposition process of several sequential depositions of different materials and the establishing of local interconnects between layers in a fashion analogous to semiconductor device manufacture, or indeed multilayer printed circuit boards.